The dynamic bandwidth transmission scheme has been adopted in the IEEE 802.11n standard to allow a transmitter of a 40 MHz BSS (basic service set) to transmit either 20 MHz or 40 MHz signal depending on clear channel assessment (CCA) sensing. For the upcoming IEEE 802.11ac standard, significant wider channel bandwidths (20 MHz, 40 MHz, 80 MHz, and 160 MHz) were proposed. The dynamic bandwidth transmission scheme in IEEE 802.11n is extended for the wider channel bandwidth in IEEE 802.11ac. For a BSS of certain bandwidth, a valid transmission sub-channel shall have bandwidth, allowable in the IEEE 802.11ac, equal to or smaller than the full bandwidth of the BSS and contains the designated primary sub-channel of the BSS. Based on the CCA sensing in the valid transmission bandwidths, the transmitter is allowed to transmit in any of the valid transmission sub-channels as long as the CCA indicates the sub-channel (or full channel) is idle. This dynamic transmission bandwidth scheme allows system bandwidth resource to be efficiently utilized.
While the transmitter is making decision regarding the selection of the transmission sub-channel, other transmission parameters such as modulation and coding scheme (MCS), precoding, and transmit power might need to be changed based on the sub-channel conditions. For example, fast link adaptation can adapt MCS according to time varying channel conditions to increase throughput of a system. Fast link adaptation can be supported via a request and feedback process. Although the transmitting device does not have to depend on MCS feedback, it is widely adopted by the implementers. The transmitting device sends a MCS request and a sounding signal and the receiver sends the MCS feedback. Since different receiver implementations can have different receiver sensitivity levels, the receiver typically can make more accurate decision regarding the appropriate MCS to be used based on the channel conditions.
FIG. 1 (Prior Art) illustrates a conventional sounding and feedback process in a wireless system 10. Each channel sounding and feedback process is followed by a series of MIMO frame exchange. During channel sounding and feedback, a transmitting device (initiator 101) sends a sounding announcement (e.g., null data packet announcement (NDPA) 103,) followed by a sounding packet (e.g., null data packet (NPD) 104,) to a receiving device (responder 102) participating in the process. The responder estimates the channel during the preamble portion of the sounding packet. The responder then feedbacks the average SNR (signal-to-noise ratio) and CSI (channel state information) to allow the initiator to compute the transmit antenna (precoding) weights for MIMO transmission. Feedback packet 105 may also include other channel quality metrics such as BER, SNR/SINR, and mutual information. More specifically, the responder determines an appropriate MCS for the current channel and feedbacks the MCS to the initiator for fast link adaptation.
Feedback of accurate channel quality information such as SNR and MCS allows the transmitter to make correct decision regarding transmission bandwidth adjustment as well as MCS adaptation to improve system performance. In current implementation, channel quality information is provided based on a fixed sub-channel (e.g., the sounding bandwidth) and obtained through a sounding and feedback protocol. The channel conditions, however, could be significantly different in different sub-channels due to frequency selective fading. To have channel quality information for all valid sub-channels, multiple requests and feedbacks are required. This leads to increased system overhead.
Moreover, the transmission bandwidth adjustment is made at the transmitter using the CCA sensing immediately prior to transmission. In contrast, the channel sounding and feedback for transmit beamforming and MCS adaptation is a slow process. It is thus possible that the transmission bandwidth is regularly less than the sounding bandwidth. A solution is sought to allow dynamic transmission bandwidth and fast link adaptation to be executed more efficiently and accurately.